The present invention relates to an improvement on the intermediate transfer member that is used in the electrostatic recording process for electrophotographic devices and electrostatic recording devices such as copying machines and printers in which an electrostatic latent image is formed on the surface of an image-forming body, the latent image is developed with a toner, and the toner image is transferred to a recording medium such as paper via said intermediate transfer member which holds the toner image temporarily before image transfer. The present invention relates also to an improvement on the image-forming device which forms images by the intermediate transfer system using said intermediate transfer member.
Conventional electrophotographic image-forming devices such as copying machines and printers work in the following manner. First, a photosensitive body (image-forming body) has its surface uniformly charged by a charging means such as charging roll. The charged photosensitive body is given a projected image through an optical system, so that the part struck by light becomes discharged, thereby forming an electrostatic latent image. The photosensitive body is given a toner by the aid of a developing roll or any other developing means, so that the latent image electrostatically attracts the toner, thereby forming a toner image. The toner image is transferred to a recording medium such as paper by a transfer means such as a transfer roll. The transferred image is fixed with heating to the recording medium by the aid of a fixing means such as a fixing roll. In this way a print image is obtained.
The above-mentioned process is basically applied also to color printers and color copying machines; however, it needs some modifications for color images which are formed by four toners, that is, magenta, yellow, cyan, and black. In order to obtain a desired hue, a process is necessary to overlay these toners in a certain ratio. There have been proposed several systems to achieve this object.
The first system is the multiple development system, in which the electrostatic latent image on the photosensitive body is developed (made visible) by sequentially overlaying the four toners, magenta, yellow, cyan, and black so that a color toner image is formed on the photosensitive body as in the case of monochromatic printing. This system permits the device to be constructed comparatively compact but has the disadvantage of having difficulties in tone control and being unable to produce high-quality images.
The second system is the tandem system, in which four photosensitive bodies are arranged in tandem and latent images on them are developed with four toners (magenta, yellow, cyan, and black) respectively. The resulting toner images are transferred sequentially to a recording medium such as paper so as to reproduce a color image thereon. This system gives rise to images of good quality but has the disadvantage of requiring a large, expensive device because each photosensitive body has a charging mechanism and a developing mechanism.
The third system is the transfer drum system, in which a recording medium such as paper is wound around a transfer drum which is turned four times such that the four magenta, yellow, cyan, and black toner images on the photosensitive body are sequentially transferred to the recording medium so as to reproduce a color image. This system produces images of comparatively good quality but has the disadvantage of involving difficulties in winding a thick paper such as post card around the transfer drum. This restricts the kind of recording medium to be used.
In order to eliminate the above-mentioned disadvantages of the multiple development system, the tandem system, and the transfer drum system, there has been proposed a new system called the intermediate transfer system. This system does not need a large device, nor does it restrict the kind of recording medium to be used.
The intermediate transfer system is designed such that the toner images of magenta, yellow, cyan, and black on the photosensitive body are sequentially transferred to and held temporarily by an intermediate transfer member in the form of drum or belt so that a color image is formed on the intermediate transfer member, and this color image is finally transferred to a recording medium such as paper. This system has the advantage of producing a high-quality image because of its ability to control the tone by overlaying four toner images. This system does not need a large device because it obviates photosensitive bodies arranged in tandem as in the tandem system, nor does it restrict the kind of recording medium to be used because it obviates the necessity of winding the recording medium around the transfer drum as in the transfer drum system.
The color image formation by the intermediate transfer system may be accomplished by a device shown in FIG. 1 (which employs a cylindrical intermediate transfer member) or by a device shown in FIG. 2 (which employs a belt-like intermediate transfer member).
Referring to FIGS. 1 and 2, there is shown a cylindrical photosensitive body 1 which rotates in the direction of the arrow indicated. This photosensitive body 1 is charged by the primary charger 2. Upon exposure to the image light 3, the exposed part loses the charge and hence forms on the photosensitive body 1 an electrostatic latent image corresponding to the first color component. The electrostatic latent image is developed with the first color (magenta toner M) by the developer 41. There is formed on the photosensitive body 1 an image of the first color (magenta toner). This toner image is transferred to the intermediate transfer drum 20a (in FIG. 1) or the intermediate transfer belt 20b (in FIG. 2) which turns in contact with the photosensitive body 1. (The drum and belt are collectively referred to as "intermediate transfer member 20a or 20b" hereinafter.) The transfer from the photosensitive body 1 to the intermediate transfer member 20a or 20b takes place at the nip between them because the latter is biased by the power source 61. After the transfer of the first toner (magenta) image to the intermediate transfer member 20a or 20b, the photosensitive body 1 has its surface cleaned by the cleaning unit 14. This cycle completes the development and transfer operation as the photosensitive body 1 makes one turn. Subsequently, the photosensitive body 1 turns three times and the toner images of the second color (cyan), the third color (yellow), and the fourth color (black) are formed consecutively on the photosensitive body 1 by the developing units 42, 43, and 44. After each turn, each toner image is transferred to the intermediate transfer member 20a or 20b and overlaid on the previously transferred image. Thus a synthesized color toner image corresponding to the original color image is formed on the intermediate transfer member 20a or 20b. Incidentally, the device shown in FIG. 2 has a developing station which holds developers 41 to 44 which are displaced sequentially each time the photosensitive body 1 turns so that development takes place sequentially with magenta toner (M), cyan toner (C), yellow toner (Y), and black toner (B).
With an image of overlaid color toners formed thereon, the intermediate transfer member 20a or 20b comes into contact with the transfer roller 25, and the nip between them receives a recording medium 24 (such as paper) from the paper feed cassette 9. At the same time, a secondary transfer bias is applied to the transfer roller 25 from the power source 29, so that the synthesized color toner image is transferred from the intermediate transfer member 20a or 20b to the recording medium 24. The recording medium 24 is further led to the fixing station 15 in which the synthesized color toner image is fixed by heating on the recording medium 24. After transfer, the intermediate transfer member 20a or 20b has its surface cleaned of residual toner by the cleaning unit 35 and returns to the initial state and becomes ready for the next cycle of image forming.
For images to be formed by the intermediate transfer system, the above-mentioned intermediate transfer member 20a or 20b should be made of a soft, resilient material because it comes into direct contact with the photosensitive body 1, the toner image, and the recording medium 24. A cylindrical intermediate transfer member 20a as shown in FIG. 1 is made up of a metal drum base (or cylindrical metal core) 200 and a surface layer of electrically conductive rubbery elastic material 201 as shown in FIG. 3. A belt-like intermediate transfer member 20b as shown in FIG. 2 is a belt formed from an electrically conductive rubbery elastic material 201 reinforced with fabrics or spirally wound yarns (not shown) as shown in FIG. 4. The rubbery elastic material 201 may have on its surface a resin coating layer 202 to protect the photosensitive body from being stained, to prevent the toner from sticking to it, and to decrease the coefficient of friction.
When used for an electrophotographic image-forming device, the intermediate transfer member shown in FIGS. 1 and 2 greatly affects the transfer efficiency and hence the image forming depending on its resistivity. It is desirable that the intermediate transfer member have a resistivity in the so-called middle range of 10.sup.11 to 10.sup.14 .OMEGA..multidot.cm. This presents difficulties in making the intermediate transfer member free from variation in resistance. It is particularly difficult to adjust a rubber compound to such a middle range of resistivity. An idea proposed so far to address this problem is that the layer of rubbery elastic body 201 is adjusted to a resistivity of 10.sup.3 to 10.sup.6 .OMEGA..multidot.cm, which is comparatively easy to attain, and this layer is covered with a coating layer 202 of resin having a comparatively high resistivity, so that the intermediate transfer member as a whole has a desired resistivity in the middle range as mentioned above.
The disadvantage of this layer structure is that the resin coating layer 202 is generally much harder than the rubbery elastic body and hence it does not fully conform to the elastic deformation of the electrically conductive rubber layer 201. The result is that the coating layer 202 formed on the intermediate transfer drum 20a or the intermediate transfer belt 20b tends to crack. The cracking causes rubber compounding ingredients to ooze out, catches toner particles, or changes the coefficient of friction. These troubles defeat the desired object. The durability of the coating layer is important particularly for the belt-like intermediate transfer member which undergoes extreme flexing.
A conceivable solution to this problem is to make the coating layer 202 from a soft resin; however, this is not practical because a soft resin is subject to large plastic deformation and is sticky with a high coefficient of friction.
When the coating layer 202 is to function to adjust the resistivity of the intermediate transfer member as a whole, it has to be incorporated with an electrically conducting material such as carbon black for decrease in resistivity because a resinous material in general has a high volume resistivity. This leads to the necessity of complex processes, including preparation of a coating material and formation of a coating layer, both containing an electrically conducting material uniformly dispersed therein.